Abstract : This paper proposes an analog-to-digital converter with two working modes. In the first mode, the system is a sigma-delta passive converter: the analog modulator uses a passive switched-capacitor low-pass filter and the only active element is the comparator. The consumption is low and the resolution is moderate (9 bits). In the second mode, the expected resolution is 15 bits. For that, the passive sigma-delta modulator is put in a loop with a low-pass amplifier and some digital processing elements. The principle of this two-mode system is validated by functional simulations and by the test of a circuit realized in a 0.35μm CMOS technology.